1. Field of the Invention
This invention relates to a system for monitoring the transmission capability of electrical transmission lines. The transmission capability of power lines will vary with the temperature of the line because the current carrying capacity is limited by the allowable sag of the line between transmission towers. In order to adequately control the load on a line it is necessary to know the condition of the line in terms of the sags of its spans. The present invention provides a system for determining maximum permissable power line current.
2. Description of the Prior Art
A major problem in overhead power transmission lines is that of clearance between the line and the nearest point on earth. If the line becomes too close to the earth or adjacent structures, problems can rise from electric flashover from the power line which can cause extensive electrical damage. For this and other reasons it is necessary to limit the current of these transmission lines so than any undue sag may be prevented. The major cause of increased sag is that of heating of the line. As the temperature increases the power line expands and clearance problems arise.
The heating of the line results from various sources. One cause is the heat that is generated in the conductor by the electrical current flowing through it which causes I.sup.2 R heating in the conductor. Thus, as the current in the line increases there is a greater generation of heat with the resulting increase in line sag. Line heating is also effected by solar heat and ambient temperature in the surrounding area. This heating of the line is offset by the cooling effect of wind passing over the line and heat radiated from the conductor.
Traditionally, transmission lines were rated based on an assumed combination of worst cooling conditions, consisting of a combination of expected highest ambient temperature, solar radiation, and low speed. Such traditional current ratings were highly conservative. To take advantage of this conservatism, methods have been developed to either monitor some of the cooling conditions or the actual temperature of the conductor and to adjust current ratings and on such monitored data.
In the prior art, a number of methods have been used for determining line temperature. A first such method is that of theoretical calculation. Assumptions are made of wind speed and direction, ambient temperature and solar radiation and calculations are made for arriving at the line condition. Because the calculations are based on theoretical assumptions, the result can be at considerable variance from the actual line condition which might permit greater line current than exists or on the other hand, dictate a lower actual line current.
In some instances, weather stations have been established in the general location of the transmission linen in order to monitor the weather to thus provide somewhat more reliable data that is then used to calculate the line conditions including the temperature of the line. A third method for monitoring the line known to the prior art, is to provide sensor devices mounted on the conductor along the length of the line at various intervals to measure conductor temperatures from which load capacity can be determined. These various systems of the prior art are disclosed, for example, in U.S. Pat. Nos. 4,268,818 and 4,420,752, and 4,806,855. These later monitors have been somewhat more effective in identifying actual temperatures of the conductors.
Additional systems are disclosed in applicant's U.S. Pat. No. 5,235,861 and pending applications Ser. No. 08/251,242, filed May 30, 1994.